The UK's winter flu season has started five weeks earlier than usual, health officials have warned.
The UK's Health Safety Agency said cases were rising rapidly among children and young people and warned the virus would soon begin to spread among older age groups.
The organization urged people eligible for a flu vaccine to seek protection.
However, they said it is too early to tell how difficult or severe this year's flu season will be.
The past three years have seen the two worst winter flu seasons in a decade, partly due to the virus returning after Covid restrictions and when immunity was low.
Last year, about 8,000 people died from the flu, and the 2022-2023 flu season has seen nearly 16,000 deaths.
UKHSA head of influenza Dr Suzanne McDonald said: “We are concerned that flu has started to spread a little earlier than usual.
“This is a very serious disease. It's not just a bad cold.
“We started to notice early activity, especially in young people, mainly university-aged students, but also school-aged children.
“We expect this to begin to spread throughout the community in the coming weeks and we will be monitoring this very closely.”
Dr Alex Allen, UKHSA consultant epidemiologist, urged all eligible people to come forward for their vaccination.
Anyone aged 65 or over, pregnant women and people with certain chronic conditions can get a free flu jab on the NHS.
“If you are eligible for the vaccine, it is because you are at greater risk of severe illness – every year we see many thousands of people in hospital and too many deaths from flu.
“It is important that everyone who is eligible makes an appointment as soon as possible. The vaccine is our best defense.”
It also reminded parents of children under 16 to return consent forms to school so their children can be vaccinated, or, if they are two and three years old, to arrange a vaccination with their GP.
“The nasal spray vaccine can help prevent your child from becoming seriously ill and reduce the risk of spreading flu to vulnerable family members.
“Anyone who has flu or Covid symptoms, including a high temperature, cough, feeling tired or in pain, should minimize contact with other people, especially vulnerable people.
“If you need to go outside with symptoms, consider wearing a mask, washing your hands regularly and keeping indoor areas well ventilated,” he added.
Meanwhile, the UKHSA said Covid cases were falling and virus levels were now at baseline.
